The proposed research seeks to develop in vitro models of blood molecules that may be involved in this process by i) determining how murine endothelial cells derived from transgenic mouse tumors model and interact with normal endothelial cells; ii) determining if cystic embryoid bodies that are differentiated in vitro from mouse embryonic stem cells mimic the mouse yolk sac in the expression pattern of various molecules, iii) determining the sensitivity of in vitro vascular development to exogenous factors, and iv) identifying novel genes that may be involved in vascular development.